The present invention relates to drilling apparatus used in the exploration for petroleum products and more specifically to a rotary drill bit for such apparatus having conically shaped rotary cutters which are supported by a plurality of bearings on the drill bit body.
Typically the bearing arrangement supporting the cutter on the drill bit body includes friction bearings adjacent the tip of the body and rollers remote from the tip. These bearings are usually lubricated by filling the common bearing cavity which is sealed at its outer end with a single lubricant. An arrangement of this type is illustrated in U.S. Pat. No. 3,127,942. The seal which seals the bearing cavity at the roller end is commonly a sliding seal ring which tends to wear during operation of the rotary drill bit so that after a predetermined operation period, leakage losses of the lubricant occur. In the event these lubricant leakage losses are not continuously replaced with new quantities of lubricant, only a small residual amount remains in the bearing cavity. This small residual amount of lubricant is carried by centrifugal force to the end of the rotary cutter facing the rotary drill bit as the result of the rotating motion of the conical rotary cutter. Accordingly, the bearings at the opposite end near the tip of the conical rotary cutter are deprived of lubricant.
Additionally, the tip of the rotary drill bit has a higher running temperature in operation than the end facing the drill bit body which is wetted down more thoroughly and better cooled by the cooling cutting liquid in the rotary drill bit area. The lubricant filling the common bearing cavity must, therefore, be of the type designed for the very high running temperature at the end of the rotary cutter facing away from the drill bit body even though a normal lubricant, suitable for lower operating temperatures which often has a greater lubricating ability and is, therefore, more economical may be adequate for the bearing or bearings at the opposite end facing the drill bit body. Lubrication of the bearings of the known rotary drilling bit, therefore, is expensive. Furthermore, it is not optimally designed for the different running temperatures of the bearings, the rolling elements and/or the friction bearings.